In recent years, for display devices for e.g. mobile devices such as cell phones, PDA or tablet PC, touch panels, liquid crystal televisions, etc., a cover glass (protective glass) has been used in many cases to protect a display and to improve appearance. Further, in the case of e.g. a cover glass for a flat screen television such as a liquid crystal television, surface treatment may be carried out for e.g. forming a film having such a function as antireflection, prevention of impact failure, magnetic wave-shielding, near infrared ray-shielding or color-adjustment.
For such display devices, weight reduction and thickness reduction are required for differentiation by the flat screen design or for reduction of the load for transportation.
Therefore, a cover glass to be used for protecting a display is also required to be made thin. However, if the thickness of the cover glass is made thin, the strength is thereby lowered, and there has been a problem such that the cover glass itself is likely to be broken e.g. on impact by a falling or flying object in the case of an installed type or when dropped during the use in the case of a portable device, and the cover glass cannot perform the essential role to protect the display device.
In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method to form a compressive stress layer at the glass surface is commonly known.
The method to form a compressive stress layer at the glass surface, may typically be an air quenching tempering method (physical tempering method) wherein a surface of a glass plate heated to near the softening point is quenched by air cooling or the like, or a chemical tempering method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) at a glass plate surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point.
As mentioned above, the thickness of the cover glass is required to be thin. However, if the air quenching tempering method is applied to a thin glass plate having a thickness of less than 2 mm, as required for a cover glass, the temperature difference between the surface and the inside tends not to arise, and it is thereby difficult to form a compressive stress layer, and the desired property of high strength cannot be obtained. Therefore, a cover glass tempered by the latter chemical tempering method is usually used.
As such a cover glass, one having soda lime glass chemically tempered is widely used (e.g. Patent Document 1).
Soda lime glass is inexpensive and has a feature that the surface compressive stress S (hereinafter in this specification, “surface compressive stress S” may be referred to simply as “S”) of a compressive stress layer formed at the surface of the glass by the chemical tempering can be made to be at least 550 MPa, but there has been a problem that it has been difficult to make the thickness t of the compressive stress layer (hereinafter in this specification, “the thickness of the compressive stress layer” may be referred to also as “the compressive stress layer depth”) to be at least 20 μm. The glass in Example 49 given hereinafter is soda lime glass.
Therefore, one having SiO2—Al2O3—Na2O type glass different from soda lime glass, chemically tempered, has been proposed for such a cover glass (e.g. Patent Documents 2 and 3).
Such SiO2—Al2O3—Na2O type glass has a feature that it is possible not only to make the above S to be at least 550 MPa but also to make the above t to be at least 20 μm. The glass in Example 47 given hereinafter is such glass and has heretofore been used as a cover glass.